That cant be true unless the hovercraft is much wider, its simply physics. Lets assume that the board is a rectangle just to make the math easy.
The 172cm board is 30cm wide = 516cm2
if the 156 was 30cm wide it would be 468cm2, for the 156 to have as much surface area it would need to be 33.1cm wide.
It does not matter how you look at it the only way you can have increased surface area is by increasing width. There is no magic bullet here.
You can have a longer effective edge by simply taking a 172 and chopping the tail off it, the edge length has not changed although the over all length has but you have still lost surface area unless you regain it with width.
It's not on the site anymore. This is the only thing I found:
but whatever, you like long boards with swallowtails. Other people like shorter boards. rocker, taper, length, swallowtails...it's all good, they all float.
btw Terje was riding his 59 Cheetah the other day in the Nozawa Onsen trees